That depends on how much structural you wanna have it. In 3D or is 2D OK? Becuase in 2D you will reach a point where you won't have other option that to use the straight lines (already with propane). If you want real 3D structure, than you will need more than slants anyway (there are either full or dashed triangles for that).In conclusion, it is only coincidence that for ethene you can make it as it is in your book and not saying whether it is 2D or 3D. It's always better to show the angles if you can, but I would not consider your way as mistake if you wrote it into my test.